Method of studying rock mass properties and apparatus for the implementation thereof

ABSTRACT

The rock mass thermal excitation is performed by means of pumping the flush fluid through the wellbore using a tubing string, the flush fluid temperature differs from the rock mass temperature. Before the thermal excitation, during the thermal excitation and after the termination thereof differential electrical signals proportional to the well bore temperature difference are registered by at least one pair of the temperature transducers positioned along the wellbore axis. The distances between the transducers in the pairs and the number of pairs is selected in advance based on the required accuracy of the determination of the rock mass areas with different properties, minimum and maximum possible length of the rock mass areas identified and the nature and degree of the wellbore temperature noise. Differential electrical signals of the temperature transducers measured before the rock mass excitation start are compared with the differential electrical signals from the same temperature transducers measured during the thermal excitation and the differential electrical signals of different temperature transducers positioned along the wellbore are compared with one another. Based on the comparison results of the differential electrical signals the difference of different rock mass areas are characterized by their properties and the boundaries between the rock mass areas with different properties are identified.

FIELD OF THE INVENTION

The invention is related to the studies of rock mass and processestherein by means of wellbore measurements, particularly to obtaining therock mass data by means of wellbore temperature measurements. Examplesof such measurements may be temperature measurements in the well drilledin the rock mass with the oil reservoir when the rock mass thermal modeis changed by its cooling or heating through injecting flush mud withthe temperature different from the rock mass temperature with subsequenttemperature record in the well at different depths. By the recorded rateof the rock mass temperature change in different layers after flushingzones with different oil content are identified. In the latter case suchmeasurements are used, for example, to improve oil fields' developmentefficiency. Another application area of the invention is related to thestudies of the rock mass properties being reservoirs with geothermalfluid and vapor. The invention may be applied to study rock masses withthe ore mineral deposits.

BACKGROUND OF THE INVENTION

A method of rock mass properties studies consisting in the record of thetemperature distribution along the well drilled in the rock mass afterthe rock mass thermal excitation by pumping through the flush mud withthe temperature different from that of the rock mass and subsequentidentification of productive layers with different properties, thismethod is described in: V. N. Dakhnov Promyslovaya Geofizika (ProductionGeophysisc), Moscow, Gostopizdat, 1959, pp. 56-59. In this case theproductive layers' areas are classified by the temperature difference inthe productive reservoirs' zones relative to the rock massnon-productive areas after the flushing completion. A dramaticdisadvantage of this method is the difficulty in identifying theproductive reservoirs' zones when these zones are separated from thewell by the layers with the flushing mud due to the impossibility torecord small temperature drops between productive layers andnon-productive areas against the background of large absolute values ofthe temperature characteristic for the rock mass at the oil occurrencedepths. Another disadvantage of this method is the fact that thetemperature in the wellbore is recorded after the flushing fluid pumpingthrough the well when the temperature profile at the boundaries of therock mass layers with different properties is blurred due to conductiveheat transfer process occurring between the rock mass sections withdifferent temperature. Yet another disadvantage of this method is thefact that the time period during which the wellbore temperature profileshould be recorded is not defined which reduces the method applicationefficiency. The method's disadvantage also consists in the fact that therock mass temperature change record as function of time at each depthselected is impossible which also reduces the method's efficiencybecause it prevents segregating the effects of the interference and rockmass properties by the different degree of the manifestation thereof interms of temperature changes as function of time.

Apparatus for the rock mass properties' study consisting of flushingfluid injection unit, temperature transducer, temperature transducersignal electronic record and processing unit, cylindrical body in whichthe temperature transducer and electronic unit are located, cable forthe cylindrical body sinking into the wellbore, power supply andmeasurement results data transfer to the surface, the apparatus isdescribed in: V. N. Dakhnov Promyslovaya Geofizika (ProductionGeophysisc), Moscow, Gostopizdat, 1959, pp. 56-59. The device is usedfor the implementation of the method of rock mass properties studydescribed in the same book. A disadvantage of this apparatus is theimpossibility to record the wellbore temperature distribution during theflushing mud injection into the well which results, as mentioned before,to the extended measurement time, indistinct identification of theboundaries of the rock mass strata with different properties. Anotherdisadvantage is the availability of one temperature transducer onlywhich results in the insufficient sensitivity of the rock mass layerswith low difference in their properties.

SUMMARY OF THE INVENTION

The engineering result attained in case of the implementation of theinvention claimed consists in the enhanced accuracy and efficiency ofthe identification of the rock mass zones with different thermalproperties. The method provides the segregation of the rock mass locatedalong the wellbore into the zones with different thermalproperties—thermal conduction, thermal diffusivity and volumetric heatcapacity. Such segregated zones may include, for example, zones withrock water- or oil-saturation, or zones with different degree of therocks oil-saturation. All these zones may be identified using theengineering solution claimed because they are characterized withdifferent thermal properties.

The said engineering result is attained due to thermal excitation of therock mass or a portion thereof by means of pumping the flushing fluidthrough the wellbore using a tubing string (the flush fluid temperaturemust differ from the rock mass temperature), subsequent record of thewellbore temperature changes at least in one section thereof, andidentification of the rock mass areas with different thermal propertiesbased on the measurement results. Prior to the start of the thermalexcitation of the rock mass (or a portion thereof) and after thecompletion of the excitation, continuously or intermittently at theintervals pre-selected based on the wellbore temperature interferencenature and possible divergence of the thermal properties of the selectedrock mass areas. Differential electrical signals proportional to thetemperature difference are recorded using at least one pair oftemperature transducers located along the wellbore axis so that thetransducers' wellbore positioning depth covered the area of the rockmass in question. The distances between the transducers in the pairs andthe number of pairs shall be selected in advance based on the requiredaccuracy of the determination of the boundaries of the rock mass stratawith different properties, minimum and maximum possible length of theselected rock mass areas and wellbore temperature noise nature anddegree. The thermal excitation degree should be selected to provide therequired ratio of the differential electrical signals to the wellboretemperature noise. Differential electrical signals from the temperaturetransducers measured before the rock mass excitation start are comparedwith the differential electrical signals from the same temperaturetransducer pairs measured during the thermal excitation, anddifferential electrical signals from different temperature transducerspositioned along the wellbore are compared with one another. Based onthe results of the differential electrical signals' comparison differentrock mass areas are characterized by their properties and the boundariesbetween the rock mass areas with different thermal properties areidentified.

The method may also provide additional measurement of the flush fluidtemperature along the wellbore in the depth range in question before thethermal excitation, during the thermal excitation and after thecompletion thereof. Based on the data obtained the temperature changenature is determined, both during the rock mass thermal excitation, andduring the temperature recovery in the course of the rock masspost-excitement relaxation. Based on the data obtained the start time,interval and end time of the differential electrical signals;measurement is selected and the decision to stop the thermal excitationis made.

The thermal excitation of the rock mass or a portion thereof may beperformed periodically at pre-set duration of each thermal excitationand pauses between them or according to the harmonic law with a pre-setfrequency and intensity. Simultaneously oscillation amplitude of thedifferential electrical signals in question, their phase shift relativeto the rock mass or a portion thereof, rock mass temperature variations'amplitude and rock mass temperature variations' phase shift is measured.After that based on the measurement data set properties of the rock massvarious areas are determined.

In another embodiment the main method of the rock mass properties studyis supplemented by the fact that the periodical excitation of the rockmass or a portion thereof is performed by means of flush fluid in thetubing string with the periodical motion direction change. In this casethe tubing string lower end is located below the rock mass area inquestion so that in the rock mass area in question periodicalheteropolar flush fluid temperature change relative to the initialtemperature of the area in question took place. The frequency of thecirculating flush fluid direction change, circulating flush fluid flowrate and position of the tubing string lower end is set based on thewellbore temperature gradient to provide a sufficient amplitude of thedifferential electrical signals, hereby the amplitude of thedifferential signals in question, their phase shift relative to the rockmass (or a portion thereof) thermal excitation, rock mass temperaturevariations amplitude as well as rock mass temperature variations phaseshift relative to the thermal excitation of the rock mass or a portionthereof is measured.

Temperature transducers may be located on the tubing string, in thiscase the diameter and metallurgy of the tubing string section in whichthe transducers are positioned is selected to provide maximumdifferential electrical signals, minimum effect of the fluid convectivemotion in the gap between the production string and tubing string wallor wellbore wall on the temperature noises taking place in the gapbetween the production string and tubing string wall or wellbore wallduring the thermal excitation of the rock mass (or a portion thereof) orafter the thermal excitation stop as well as the minimum blur of thetemperature differences between the rock mass strata having differentproperties.

The main method of the rock mass properties determination may besupplemented by the fact that to provide the best segregation of therock mass areas along the wellbore based on their properties, durationand intensity of the thermal excitation of the rock mass or a portionthereof as well as differential electrical signals' measurement timesafter the start of the thermal excitation of the rock mass or a portionthereof are selected based on the dependence of the wellbore temperaturenoise on the time and based on the wellbore temperature noise in thewellbore during the measurement of the differential electrical signals'to obtain maximum ratio of the differential electrical signals to thewellbore temperature noise.

Apart from the main method, in order to enhance the accuracy of thedetermination of the boundaries between the rock mass areas along thewellbore based on their properties and reduce the vagueness of theboundaries between the rock mass areas having different properties,duration and intensity of the thermal excitation of the rock mass or aportion thereof and the time of measurement of the differentialelectrical signals after the thermal excitation start is selected basedon the nature and value of the wellbore temperature noise taking placebefore the differential electrical signals' measurements so that thespatial changes of the differential electrical signals at the areabetween the areas of the rock mass with different properties werelocalized within the minimum distance range along the wellbore.

A method of the rock mass properties may be implemented characterized bythe fact that to enhance the accuracy of the characterization of therock mass (or a portion thereof) and the characterization of theirdistributions in the direction perpendicular to the wellbore both at thestage of the thermal excitation of the rock mass or a portion thereof aswell as after the thermal excitation end the nature of the change of thedifferential electrical signals as a function of time and maximum valuesof the differential signals and the time to attain maximum values of thedifferential signals is determined, and based on the set of these valuesthe depth of the flush fluid penetration zone and oil-saturation of therock mass or a portion thereof is determined.

In the embodiment supplementary to the previous method, in order toenhance the accuracy of the characterization of the properties of therock mass or a portion thereof and characterization of the properties'distribution in the direction perpendicular to the wellbore, at leastone repeated thermal excitation of the rock mass or a portion thereof isperformed during the time different from the time of the previousthermal excitations. Every time differential electrical signals duringthe thermal excitation of the rock mass or a portion thereof aremeasured and every time maximum values of the differential signals andtime to attain maximum values of the differential signals both duringthe thermal excitation and after the completion thereof are determined.Then, based on the set of the data obtained during all the cycles of thethermal excitation of the rock mass or a portion thereof the depth ofthe flush fluid penetration zone and oil-saturation of the rock mass ora portion thereof is determined.

A method of the determination of the properties of the rock mass or aportion thereof may also be implemented in such a way that, in additionto the main method, the thermal excitation is every time performed at adifferent volume of the flush fluid injected into the well than duringthe preceding thermal excitations. Every time during or after thethermal excitation of the rock mass or a portion thereof differentialelectrical signals are measured, maximum values of the differentialelectrical signals and the times to attain maximum values of thedifferential electrical signals are found. Then, based on the set of thedata obtained during all the cycles of the thermal excitation of therock mass or a portion thereof the depth of the flush fluid penetrationzone and oil-saturation of the rock mass or a portion thereof isdetermined.

If the casing string in the wellbore is separated from the rock mass bythe cement ring, in order to improve the accuracy of the determinationof the rock mass or a portion thereof by means of the record of thetemperature noise occurring due to the changes of the cement ringthickness and the deviations of the casing string and tubing string fromthe wellbore axis, in addition to the main method, the differentialsignals are recorded at the time when the ratio of the differentialelectrical signals to the wellbore temperature noise resulting from thechanges in the cement ring thickness and deviations of the casing stringand tubing string from the wellbore axis is maximum.

In still another embodiment the main method of the rock mass properties'determination may be supplemented by the fact that in addition porosityin different parts of the roc mass alog the wellbore is determined.After that, based on the set of the results of the measurements of themaximum values of the differential signals, time of attainment of themaximum values of the differential signals and porosity the depth of theflush fluid penetration zone and oil-saturation of the rock-mass or aportion thereof are determined.

Another claimed method of the rock mass properties' determination ischaracterized by the fact that during the thermal excitation of the rockmass or a portion thereof and upon completion of the thermal excitationin the areas located in the gap between the casing string and wellborewall at different distances from the casing string differential signalsproportional to the temperature difference are additionally measured,based on the measurement results the nature and value of the temperaturenoise to be accounted for during the differential electrical signals andsubsequent rock mass properties' determination are determined.

Another method of the rock mass properties' determination is possible inwhich in addition to the main method at least one temperature transduceris displaced along the wellbore prior to the start of the thermalexcitation of the rock mass or a portion thereof and then, at leastonce, during the thermal excitation, then the temperature distributionalong the wellbore is recorded using at least one temperature transducerdisplaced along the wellbore. The temperature transducers' displacementspeed and temperature profile record start time after the start of thethermal excitation of the rock mass or a portion thereof is selected insuch a way as to provide the optimum usable signal/noise ratio. Afterthat the properties of the rock mass (or a portion thereof) aredetermined both by the temperature distribution along the wellbore andby the extent of the temperature variation in different strata of therock mass as a function of time.

Another method of the rock mass properties' determination is proposed,it is different from the main method by the fact that the temperature ismeasured at some sections along the wellbore before the start of thethermal excitation of the rock mass or portions thereof and then thetemperature is measured at some sections along the wellbore after thestart of the thermal excitation. The number of the wellbore sections inwhich the temperature is measured is every time selected to ensure therequired accuracy of the determination of the boundaries between thestrata of the rock mass with different properties. The time of thetemperature measurement along the wellbore after the thermal excitationstart is selected to provide the optimum usable signal/noise ratio.After that based on the temperature measurements at these sections alongthe wellbore prior to the start and after the start of the thermalexcitation of the rock mass or a portion thereof the wellboretemperature distribution characterizing the rock mass properties isdetermined and based on this temperature distribution the rock massstrata with different properties are determined.

Another method of the rock mass properties' determination is proposed,it is different from the main method by the fact that the differentialelectrical signals are additionally measured along one or more linesdirected along the wellbore and located parallel to each other and tothe main line along which the differential signals are measured. Thenumber of the lines and angles between them around the wellbore axis areselected proceeding from the location of the rock mass areas andwellbore space areas with different properties around the wellbore axis.

In still another method of the rock mass properties' determination inaddition to the main and preceding methods the differential electricalsignals proportional to the temperature difference in the areas locatedin the gap between the casing string and wellbore wall at differentdistances from the casing string are measured along one or more linesdirected the wellbore and located parallel to each other and parallel tothe main line along which the differential signals are measured. Thenumber of the lines and angles between them around the wellbore axis areselected proceeding from the location of the rock mass areas andwellbore space areas with potentially different properties around thewellbore axis.

For the implementation of the method above an apparatus for the rockmass properties' study is proposed, it includes a unit for the flushfluid injection into the wellbore for the thermal excitation of the rockmass or a portion thereof by the fluid circulation inside the wellbore,the unit for the adjustment of the time during which the flush fluid isinjected into the wellbore and temperature transducers positioned alongthe wellbore axis. The apparatus also includes at least one pair of thetemperature transducers providing the reception of differentialelectrical signals characterizing the temperature difference in twospots along the wellbore and a unit generating differential electricalsignals for the temperature transducer pairs ensuring the reception ofthe differential electrical signals proportional to the wellboretemperature difference in at least one pair of spots. The distancesbetween the transducers in the pairs and the number of the transducerpairs is selected based on the required accuracy of the determination ofthe location of the boundaries of the rock mass strata with differentproperties, minimum and maximum possible length of the identified rockmass areas and the degree of the temperature noise in the wellbore.Additionally the apparatus includes record unit ensuring simultaneousrecord of the differential signals measured at the same times andensuring the identification of the rock mass areas with differentproperties based on the comparison and processing of the differentialelectrical signals.

The apparatus may additionally include the unit ensuring periodicalthermal excitation of the rock mass or separate portions thereof withthe setting of a certain duration of each thermal excitation and pre-setpauses between the thermal excitations or ensuring thermal excitation onthe harmonic law with the pre-set frequency and intensity. Besides, theapparatus includes a unit providing the measurement of the oscillationsamplitude of the differential signals in question as well as a unitmeasuring the phase shift of the variations of the differential signalsin question. The apparatus also includes a unit measuring the rock masstemperature variations' amplitude and a unit measuring the phase shiftof the rock mass temperature variations relative to the thermalexcitation of the rock mass or a portion thereof.

Another embodiment of the apparatus claimed is characterized by the factthe apparatus also includes a unit ensuring periodical thermalexcitation of the rock mass or a separate part thereof by means of theflush fluid circulation in the casing string with the periodical changeof the flush fluid motion direction so that heteropolar fluidtemperature change relative to the temperature of the rock mass area inquestion took place in the said rock mass area. Besides, this apparatusalso includes a unit setting the frequency of the circulating flushfluid direction change, the circulating flush fluid flow rate and theposition of the lower end of the casing string in the wellbore based onthe wellbore temperature gradient. To determine the temperature gradientthe apparatus includes a unit evaluating the temperature gradient by thetemperature transducers' signals and the distance between thetemperature transducers along the wellbore.

Another embodiment of the apparatus for the rock mass properties' studydifferent from the main embodiment by the fact that the apparatusadditionally includes the unit for the record and amplitude-temperatureanalysis of the wellbore temperature noise, this unit is connected withthe temperature transducers. The unit for the record andamplitude-temperature analysis of the wellbore temperature noise is alsoconnected with the differential signals' matching and processing toensure the exclusion of the temperature noise with a similar frequencyfrom the differential signals in question.

A proposed supplementary embodiment of the apparatus for the rock massproperties' study differs from the main apparatus by the fact that theapparatus additionally includes temperature transducers positioned atthe same wellbore levels as the temperature transducers used for therecord of the differential electrical signals along the wellbore but atdifferent distances from the casing string in the gap between the casingstring and the wellbore wall or tubing string wall. Besides, theapparatus includes a unit providing the measurement of the differentialsignals between all the additional transducers positioned at the similardepth in the wellbore as well as a unit providing amplitude-frequencyanalysis of the differential electrical signals measured between all theadditional transducers and resulting segregation of the temperaturenoise existing in the area between the casing string and wellbore wallor tubing string wall. Besides, the apparatus includes a unit ensuringthe record and exclusion of the noise segregated from the differentialelectrical signals recorded by the pairs of the temperature transducerspositioned along the wellbore.

An apparatus for the rock mass properties' study different from the mainapparatus by the fact that the apparatus includes at least oneadditional set of temperature transducers located along the wellbore forthe measurement of the differential electrical signals similar to theset of the temperature transducers in the main apparatus is alsoproposed. The additional sets of the temperature transducers arepositioned along one or more lines directed along the wellbore andlocated parallel to one another as well as parallel to the line alongwhich the main apparatus temperature transducers for the differentialelectrical signals are located. The number of the additional temperaturetransducers and angles between the lines along which the temperaturetransducers are located around the wellbore axis is selected based onthe location of the rock mass areas and wellbore space areas withpotentially different properties.

Another embodiment of the apparatus for the rock mass properties studiesis different from the main apparatus by the fact that the apparatusadditionally includes temperature transducers whose signals are used tomeasure differential electrical signals characterizing the temperaturechanges in the wellbore in the direction from the casing string towardsthe wellbore walls, hereby these transducers are positioned on thecasing string along one or more lines directed along the wellbore andlocated parallel to each other and to the line on which the maintemperature transducers are located and along which the differentialsignals are measured, hereby the number of the lines and angles betweenthese lines along the wellbore axis are selected based on the locationof the rock mass and wellbore space areas with potentially differentproperties along the wellbore axis.

Additional embodiment of the apparatus for the rock mass properties'study is different from the main apparatus by the fact that theapparatus additionally includes a unit providing the displacement of atleast one temperature transducer and wellbore differential temperaturetransducers. Besides, the apparatus additionally includes a unitensuring setting of the displacement speed of the temperaturetransducers and wellbore differential temperature transducers, as wellas a unit ensuring the binding of each temperature transducer to thedepth for each time moment of the temperature and differentialtemperature signal. Besides the apparatus includes a unit providingperiodical change of the temperature transducers' displacement along thewellbore with the displacement direction change at the preset time.

An apparatus for the rock mass properties' study is also proposed, itdiffers from the main apparatus by the fact that the apparatusadditionally includes several temperature transducers positioned alongthe wellbore, hereby the number of the temperature transducerspositioned along the wellbore is selected to provide the requiredaccuracy of the determination of the boundaries between the rock massareas with different properties. Besides, the apparatus includes a unitfor the record and processing of the signals from the temperaturemeasurement transducers which is used to provide the temperaturemeasurement by the transducers at the pre-set time moments, temperaturedistribution record along the wellbore based on the results of thetemperature measurements after the start of the thermal excitation ofthe rock mass or a part thereof based on the temperature distributionalong the wellbore before the thermal excitation of the rock mass or aportion thereof and identification of the rock mass strata withdifferent properties.

Another claimed embodiment of the apparatus for the rock mass propertiesstudy differs from the preceding apparatuses by the fact that theapparatus includes a unit providing the flush fluid injection at apre-set flow-rate over a time unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is explained by the drawing where FIG. 1 illustrates anexample of the implementation of the claimed method of the rock massstudy.

As shown in FIG. 1, in the vertical wellbore 1 drilled in the rock mass2, in the depth range 3 (2,000-2,030 m), uniform in terms of theminerals composition and porosity separate portions 4, 5, 6,characterized by different oil-saturation need to be identified. Whereasthe thermal properties of the oil differ significantly from the thermalproperties of the water (for example, under normal pressure andtemperature conditions oil heat conductivity is 0.11-0.13 W/(m·° K), andwater heat conductivity is 0.60 W/(m·° K)), volumetric heat capacity is1.6·10⁶ J/(m³·° K) for oil and 4.2·10⁶ J/(m³·° K) for water), thedifference of separate sections 4, 5, 6 of the rock mass 2 in thermalproperties will mean the difference of separate sections 4, 5, 6 in oilsaturation. To provide thermal excitation of the rock mass 2 flush fluidis injected into the wellbore 1 through the tubing string 7, the flushfluid temperature significantly differs from the temperature of the rockmass 2 at the depth of 2,000-2,030 m in the interval 3. The sinking ofthe tubing string 7 is prepared in such a way as to provide thepost-sinking positioning of its lower end 8 at the depth of 2,050 m,i.e. below the depth of 2,030 m corresponding to the depth of relevantbottom border of the depth range 3 in question. Based on the accuracy ofthe determination of the boundaries of sections 4, 5, 6 of rock mass 2equal to 1 meter and possible length of the thermal convection cells ofthe flush fluid in the wellbore 1 of minimum 30 m 35 temperaturetransducers 9 are positioned at the tubing string 7 along its outersurface at 1-meter interval. Temperature transducers 9 are positioned atthe tubing string 7 along wellbore 1 so that their positioning depthrange was 1,998-2,032 m, i.e. covered the depth range 3, in whichsections 4, 5 and 6 of rock mass 2 with different thermal propertiesneed to be identified. Pre-calibrated resistance thermometers or opticaltemperature transducers (special fiber-optic cable for temperaturemeasurement in different points along the cable) are used as transducers9. Temperature transducers 9 are switched on in the way providing bothtemperature record in each point of the transducers 9 positioning anddifferential temperature record for different temperature transducers 9.Based on the supposed thermal properties range of the rock mass 2,diameter and depth of wellbore 1, availability of casing string 10,cement ring 11 thickness between the casing string 10 and walls of thewellbore 1, properties of the flush fluid being injected, the rate andduration of the flush fluid injection into the wellbore 1 is determinedby means of mathematical modeling—8 liters per second and 4 hours,respectively,—as well as the time duration after the completion of theflush fluid injection into the tubing string 7 during which thetemperature of sections 4, 5, 6 with different thermal properties of therock mass 2 will differ—6 hours and time interval between thedifferential temperature measurements—1 minute. The degree of thethermal excitation of the rock mass 2 in its separate part 3 is selectedto provide the required value of the ratio of the differentialelectrical signals to temperature noise electrical signals in thewellbore 1 as 50:1. Then the tubing string 7 is sunk into the wellbore 1so that its lower end 8 was positioned at the depth of 2,050 m, andbefore the start of the rock mass 2 thermal excitation the temperaturesare recorded with temperature transducers 9 and differentialtemperatures for various pairs of the temperature transducers 9. Afterthat thermal excitation of rock mass 2 is started by injecting the flushfluid into the tubing string 7 and maintaining the flush fluidtemperature at the tubing string 7 inlet constant at 15-17° C. which issignificantly lower than 69° C. recorded in the wellbore 1 before by thethermometry results. During the flush fluid injection into the wellbore1, the temperature in the wellbore 1 is recorded using temperaturetransducer 9 and temperature difference for sections 4, 5, 6 of the rockmass 2 or between the sections of the rock mass 2 located in a uniformmanner along the wellbore 1 is recorded. 4 hours later the flush fluidinjection into wellbore 1 is cut off and the record of the differentialtemperature values in the points of transducers' 9 positioning iscontinued for another 6 hours with the measurement intervals of 1 min.To account for the spatial temperature variations in the wellbore 1present in the wellbore 1 before the start of the flush fluid injectiondifferential electrical signals from different pairs of temperaturetransducers 9 located along wellbore 1 measured after the start of thethermal excitation of rock mass 2 are matched with the differentialelectrical signals of different pairs of temperature transducers 9located along wellbore 1 measured after the start of thermal excitationof rock mass 2, thus spatial variations of the temperature alongwellbore 1 eliminating the influence of the temperature spatialvariations in the wellbore 1 before the flush fluid injection start aredetermined. After that differential electrical signals of differenttemperature transducers 9 obtained after the elimination of the effectof the temperature spatial variations in the wellbore 1 before the flushfluid injection start are compared with each other. The temperaturedifference measured by differential method for one pair of temperaturetransducers 9 during the flush fluid injection or after the flush fluidinjection is equal to the value of the noise for differentialtemperature measurements, it means that both of these temperaturetransducers 9 are located at the section or sections of the rock mass 2with similar thermal properties and, consequently, the oil-saturation ofthe rocks is similar within the measurement accuracy. If the temperaturedifference measured using differential method for one pair oftemperature transducers 9 during the flush fluid injection or after theflush fluid injection exceeds the noise value for the temperaturedifferential measurements, it means that both of these transducers arepositioned in two sections (out of sections 4, 5, 6) of the rock masswith different thermal properties, and consequently, oil saturation ofthe rock mass 2 at these two sections (out of sections 4, 5, 6) isdifferent. Determination of the oil saturation and identification of thesections with a larger or smaller oil saturation is performed by meansof comparison of the differential temperature measured value and sign(plus or minus) of the differential temperature. Based on comparisonresults sections 4, 5, 6 of the rock mass 2 with different thermalproperties are identified and boundaries of these sections 4, 5, 6 withdifferent thermal properties are defined. Based on the results of thesegregation of the sections 4, 5, 6 with different thermal properties ofthe rock mass 2 sections 4, 5, 6 of the rock mass 2 with different oilsaturation are identified.

As an additional example of the implementation of the engineeringsolution proposed a case of segregating sections 4, 5, 6 with differentthermal properties of the rock mass 2 when significant depth-variablerandom deviation of the tubing string 7 axis or casing string 10 axisfrom the wellbore 1 axis (in the latter case it will result in thechange of the cement ring 11 thickness) or eccentricity of the tubingstring 7 axis or casing string 10 within the depth range 3 in whichsections 4, 5, 6 with different thermal properties which need to besegregated by means of the claimed engineering solution implementation.In both the cases significant change of the thermal resistance betweenthe walls of tubing string 7 and walls of the wellbore 1 will take placewhich will result in a significant noise in the recorded values of thetemperature and differential temperatures at different depths and,consequently, complicate the solution of the problem on segregatingsections 4, 5, 6 of the rock mass 2 with different thermal properties.To reduce this noise in the case above with the segregation of sections4, 5, 6 with different thermal properties in the depth range of2,000-2,030 m in addition 6+3

To reduce this noise in the case above with the segregation of sections4, 5, 6 with different thermal properties in the depth range of2,000-2,030 m in addition to 35 temperature transducers 9 installedalong tubing sting 7 earlier with the interval of 1 m additional setscomprising 35 temperature transducers 9 at the distance between theneighbouring temperature transducers 9 equal to 1 m. In case ofavailability of the information of equal probability of azimuthdeflections of the tubing string 7 axis and casing string 10 axisrelative to the wellbore 1 axis as well as the information of thepossibility of the tubing string 7 and casing string 10 eccentricityuniformly distributed around the axes of the tubing string and casingstring three additional sets of temperature transducers 9 are mountedalong the axis of the tubing string 7, hereby the lines of positioningof all the four sets of temperature transducers 9, each set comprising35 temperature transducers 9 is uniformly distributed along thecircumference of the tubing string 7. The process of the implementationof the proposed engineering solution differs from the previouslyconsidered case by the fact that the differential electrical signals aremeasured simultaneously along four lines oriented along wellbore 1 andlocated parallel to one another. The results of each measurement of thedifferential temperature obtained for each of the four pairs oftemperature transducers 9 at the similar depths in the wellbore 1 areaveraged. The average values of the differential temperature are used toidentify sections 4, 5, 6 of the rock mass 2 with different thermalproperties as it was done in the preceding case of the implementation ofthe engineering solution proposed.

1. A method of studying a rock mass properties comprising the steps of:thermal disturbance of the rock mass or a part thereof by means ofpumping a flush fluid through a borehole using a tubing string, theflush fluid temperature being different from that of the rock mass, thethermal disturbance degree being selected to ensure the required ratioof the differential electrical signals to the borehole temperature noiseelectrical signals, recording of differential electrical signalsproportional to the borehole temperature difference before, during andafter the thermal disturbance continuously or periodically at timeintervals of pre-selected duration determined based on the nature of thetemperature noise in the borehole and the degree of the possibledeviation of the rock mass properties which need to be determined,recording being made with the use of at least one pair of temperaturetransducers located along the borehole axis, so that the depth range ofthe temperature transducers location covered the area of the rock massin question, the distance between the transducers in the pairs and thenumber of the pairs is selected in advance based on the requiredmeasurement accuracy of the location of boundaries of rock massreservoirs with different properties, minimum and maximum possiblelength of the rock mass areas segregated as well as nature and degree ofthe temperature noise in the borehole, comparison of differentialelectrical signals from the temperature transducer pairs measured beforethe thermal rock mass disturbance with the differential electricalsignals from the same transducer pairs measured during the thermaldisturbance and differential electrical signals from differenttransducer pairs located along the borehole axis, characterizingdifferences of various rock mass areas with different properties basedon the results of the differential electrical signals' comparison; andidentifying the boundaries between the rock mass areas with differentthermal properties.
 2. The method of claim 1 wherein additionally fluidtemperature along the wellbore in the depth range in question or in aseparate part of the wellbore in the depth range in question is measuredbefore the thermal excitation, during the thermal excitation and afterthe cancellation thereof, and by the data obtained the temperaturechange nature both during the thermal excitation and during thetemperature recovery in the course of the rock mass relaxation after thethermal excitation is determined, based on the data obtained the starttime, intervals and ending time of the differential electrical signals'measurements are determined and the decision of the thermal excitationtermination is made.
 3. The method of claim 1 wherein the thermalexcitation of the rock mass or a separate part thereof is performedperiodically at pre-set duration of each thermal excitation and pausesbetween them and simultaneously measuring the amplitude of theoscillations of the differential signals in question, their phase shiftrelative to the thermal excitation of the rock mass or a separate partthereof, amplitude of the oscillations of the rock mass temperature andphase shift of the rock mass temperature oscillations' temperature. 4.The method of claim 1 wherein the thermal excitation of the rock mass ora part thereof is performed according to the harmonic law at a presetfrequency and intensity and simultaneously the oscillation amplitude ofthe differential signals in question, their phase shift relative to thethermal excitation of the rock mass or a part thereof, rock masstemperature oscillations amplitude and rock mass temperatureoscillations phase shift is measured.
 5. The method of claim 1 whereinthe periodical thermal excitation of the rock mass or a part thereof isperformed by means of the flush fluid circulation in the tubing stringwith the periodical change of the flush fluid motion direction, herebythe lower end of the tubing string is located lower than the rock masszone in question so that flush fluid temperature periodical changerelative to the temperature of the rock mass zone in question took placein the rock mass zone in question, hereby the circulating flush fluiddirection change intervals, circulating flush fluid flow rate andposition of the tubing string lower end in the wellbore is set based onthe wellbore temperature gradient to provide sufficient amplitude of thedifferential electrical signals.
 6. The method of claim 1 wherein thetemperature transducers are located at the tubing string and thediameter and material of the tubing string section at which thetemperature transducers are located are selected to provide maximumdifferential electrical signals, minimum effect of the fluid convectivemotion in the gap between the tubing string and casing string wall orwellbore wall onto the temperature noises occurring in the gap betweenthe tubing string and casing string wall or wellbore wall during thethermal excitation of the rock mass or a separate part thereof or afterthe termination of the thermal excitation as well as minimum blurring ofthe temperature boundaries between the rock mass strata with differentproperties.
 7. The method of claim 1 wherein the duration and intensityof the thermal excitation of the rock mass or a separate part thereofand the times of the differential electrical signals' measurement duringthe thermal excitation are selected based on the wellbore temperaturenoise as a function of time and based on the wellbore temperature noisevalue existing at the times of the differential electrical signalsmeasurement to obtain maximum ratio of the differential electricalsignals to the wellbore temperature noise.
 8. The method of claim 1wherein the duration and intensity of the thermal excitation of the rockmass or a separate part thereof and the times of the differentialelectrical signals' measurement during the thermal excitation areselected based on the nature and value of the wellbore temperature noisepresent by the time of the differential electrical signals measurementso that the spatial measurements of the differential electrical signalsat the section between the rock mass areas with different propertieswere localized in the minimum distance range along the wellbore.
 9. Themethod of claim 1 wherein during the thermal excitation of the rock massor a part thereof and after the termination of the thermal excitationthe nature of changes of the differential electrical signals as functionof time, maximum values of the differential electrical signals and thetime to attain maximum values of the differential signals are determinedand based on the set of these values the flush fluid penetration zonedepth and oil saturation or the rock mass or a separate part thereof aredetermined.
 10. The method of claim 1 wherein at least once a repeatedthermal excitation of the rock mass or a separate part thereof isperformed and each repeated thermal excitation is performed over theperiod different from the previous thermal excitations duration,differential electrical signals are measured after each repeated thermalexcitation and every time maximum values of the differential signals andthe times of the differential signals maximum values attainment aredetermined both during the thermal excitation and upon the terminationthereof; after that, based on the set of the data obtained in all thecycles of the thermal excitation of the rock mass or a part thereof theflush fluid penetration zone depth and oil saturation or the rock massor a separate part thereof are determined.
 11. The method of claim 10wherein during each repeated thermal excitation the flush fluid ispumped through the wellbore in the volume different from that used inprevious thermal excitations.
 12. The method of claim 1 wherein in caseof the presence of the casing string separated from the rock mass by acement ring in the wellbore the differential electrical signals arerecorded when the ratio of the differential electrical signals to thetemperature noise in the wellbore due to the cement ring thicknessvariations of the cement ring thickness and deviations of the tubingstring and casing string from the wellbore axis is maximum.
 13. Themethod of claim 1 wherein additionally porosity in different parts ofthe rock mass along the wellbore is determined, after which the flushfluid penetration zone and oil saturation of the rock mass or a separatepart thereof are determined based on the set of the results of themeasurements of the maximum values of the differential signals,differential signals maximum attainment time and porosity.
 14. Themethod of claim 1 wherein during the thermal excitation of the rock massor a separate part thereof and after the thermal excitation completionin the areas located in the gap between the tubing string and wellborewall at different distances from the tubing string additionallydifferential signals proportional to the temperature difference aremeasured and based on the measurement results the nature and value ofthe temperature noise are determined which are taken into account duringthe differential electrical signals processing and subsequentdetermination of the rock mass properties.
 15. The method of claim 1wherein at least on temperature transducer is displaced along thewellbore before the thermal excitation start and then at least onceduring the thermal excitation; and the temperature distribution alongthe wellbore is recorded using at least one temperature transducerdisplaced along the wellbore hereby the temperature transducersdisplacement speed and the moment of the temperature profile record timeafter the thermal excitation start is selected to provide optimum usablesignal/noise ratio.
 16. The method of claim 1 wherein additionallytemperature at several sections along the wellbore before the thermalexcitation of the rock mass or separate parts thereof is measured, thenthe temperature at several sections along the wellbore during thethermal excitation is measured, hereby the number of the sections alongthe wellbore every time is selected to provide the required accuracy ofthe determination of the boundaries between the rock mass strata withdifferent properties, moments of the temperature measurement along thewellbore are selected to provide the optimum usable signal/noise ratioand based on the measurement results the temperature distribution alongthe wellbore characterizing the rock mass properties by which the rockmass strata with different properties are determined.
 17. The method ofclaim 1 wherein additional measurements of the differential electricalsignals along one or more lines oriented along the wellbore and locatedparallel to one another and parallel to the main measurement line areperformed, hereby the number of the lines and angles between these linesaround the wellbore axis are selected based on the location of the rockmass and wellbore space areas with potentially different properties. 18.An apparatus for studying a rock mass properties comprising: a unitensuring a flush fluid injection into a borehole to perform thermaldisturbance of the rock mass or a part thereof by circulating theborehole fluid, temperature transducers disposed along the boreholeaxis, at least one temperature transducer pair providing obtainingdifferential electrical signals characterizing the temperaturedifference in two points along the bore hole, a unit generatingdifferential electric signals for temperature transducer pairs andproviding reception of the differential electric signals proportional tothe difference of the borehole temperature in at least one pair ofpoints, the distances between the transducers in the pairs and thenumber of the transducer pairs are selected based on the requiredaccuracy of the location of the boundaries of the rock mass strata withdifferent properties, minimum and maximum possible length of thesegregated rock mass areas as well as the nature and degree of thetemperature noise in the well, a unit adjusting the flush fluidinjection time, a record unit ensuring simultaneous record of all thedifferential signals from the temperature transducers and a unit for thecomparison and processing of the differential signals measured at thesame time moments ensuring the segregation of the rock mass areas withdifferent properties based on the comparison results and processing ofdifferential electrical signals.
 19. The apparatus of claim 18 furthercomprising a unit providing periodical thermal excitation of the rockmass or separate parts thereof with setting a certain duration of eachthermal excitation and certain pauses between the thermal excitations orproviding thermal excitation as per harmonic law with the presetfrequency and intensity.
 20. The apparatus of claim 18 furthercomprising a unit providing the measurements of the oscillationsamplitude of the differential signals in question, a unit measuring thephase shift of the oscillations amplitude of the differential signals inquestion, as well as a unit measuring the phase shift of the rock masstemperature oscillations relative to the thermal excitation of the rockmass or a separate part thereof.
 21. The apparatus of claim 18 furthercomprising a unit providing periodical thermal excitation of the rockmass or a separate part thereof by means of circulating the flush fluidin the tubing string with the periodical change of the flush fluidmotion direction so that periodical change of the flush fluidtemperature relative to the temperature of the rock mass in questiontook place, a unit setting the intervals of the circulating flush fluiddirection change, circulating flush fluid flow rate and position of thelower end of the tubing string in the wellbore based on the wellboretemperature gradient as well as a unit evaluating the temperaturegradient by the signals from the temperature transducer and the distancebetween the temperature transducers along the wellbore.
 22. Theapparatus of claim 18 further comprising a unit for the record andfrequency amplitude analysis of the wellbore temperature noise connectedwith the temperature transducers and the unit for matching andprocessing of the differential signals to cut the temperature noise withthe similar frequency out of the differential signals in question. 23.The apparatus of claim 18 further comprising the temperature transducerslocated at the same wellbore levels as the temperature transducers usedto record the differential electrical signals along the wellbore in thegap from the tubing string to the wellbore wall or casing string wall aswell as a unit measuring the differential signals between all theadditional transducers positioned at a similar wellbore depth, a unitproviding amplitude frequency analysis of the differential electricalsignals measured between all the additional transducers, and theresulting segregation of the temperature noise existing in the spacebetween the tubing string and wellbore wall or casing string wall, and aunit for the record and elimination of the segregated noise from thedifferential electrical signals recorded by the pairs of the temperaturetransducers located along the wellbore.
 24. The apparatus of claim 18further comprising at least one additional set of the temperaturetransducers positioned along the wellbore to measure differentialelectrical signals, the transducers are positioned parallel to the linealong which the temperature transducers used for the differentialelectrical signals measurement are positioned and along one or morelines oriented along the wellbore and located parallel to one another,as well as the main apparatus signals; the number of the additionaltemperature transducers' sets and the angles between the lines alongwhich the temperature transducers in the additional sets are positionedaround the wellbore axis are selected based on the location of the rockmass areas and wellbore space areas with potentially differentproperties.
 25. The apparatus of claim 18 further comprising thetemperature transducers whose signals are used to measure differentialelectrical signals characterizing wellbore temperature change in theradial direction from the tubing string to the wellbore walls, herebythese transducers are positioned at the tubing string along one or morelines oriented along the wellbore and located parallel to one anotherand parallel to the location line of the main temperature transducersalong which the differential signals are measured, hereby the number ofthe lines and angles between these lines around the wellbore axis areselected based on the location of the rock mass and wellbore space withpotentially different properties around the wellbore axis.
 26. Theapparatus of claim 18 further comprising a unit displacing at least onetemperature transducer and differential temperature measurementtransducer along the wellbore, a unit setting a certain speed of thetemperature transducers and differential temperature measurementtransducers displacement along the wellbore as well as a unit linkingeach temperature transducer to the depth for each temperature anddifferential temperature signal record time, and a unit providingperiodical change of the temperature transducers' displacement along thewellbore with the direction change at a preset time.
 27. The apparatusof claim 18 further comprising several temperature transducerspositioned along the wellbore, the number of the transducers is selectedto provide the required accuracy of the determination of the boundariesbetween the rock mass strata with different properties as well as a unitfor the record and processing of the signals from the transducers, theunit is used for the transducers' temperature measurements at pre-settime moments, record the temperature distribution along the wellborebased on the results of the temperature measurements after the start ofthe thermal excitation of the rock mass or a part thereof and identifythe rock mass strata with different properties.
 28. The apparatus ofclaim 18 further comprising a unit injecting the flush fluid through thewellbore at the required volume over a time unit.